As problems such as global warming due to emission of CO2 and the like by use of fossil fuel and radioactive contamination due to accidents at nuclear power plants or radioactive waste are becoming increasingly serious in recent years, there has been a growing interest in global environment and energy. Under these circumstances, solar power generation that uses sunlight as an unlimited source of clean energy, geothermal power generation that uses geothermal heat, wind power generation that uses wind power, and the like have been put into practical use worldwide.
There are various forms of solar power generation using solar cells in correspondence with output scales ranging from several W to several thousand kW. In a typical solar power generation system using solar cells, a DC power generated by the solar cells is converted (DC/AC-converted) into an AC power by an inverter or the like and supplied to the loads of subscriber's houses or a commercial AC power system (to also be simply referred to as a “system” hereinafter).
FIG. 2 is a view showing the schematic arrangement of a conventional general solar power generation system. As shown in FIG. 2, in a general solar power generation system 8, a plurality of solar cells are connected in series to form a solar battery module 6 as a unit. A plurality of solar battery modules 6 are further connected in series to construct a solar battery string 7 (also called a solar battery array). In addition, a plurality of solar battery strings 7 are connected in parallel to form a solar battery array. A DC power from the solar battery array is collected by a collection box 9. The collected power is converted into an AC power by an inverter 3 and connected to a load 4 or system 5.
In the solar power generation system 8, if the outputs from the plurality of solar battery strings 7 are different due to the influence of a variation in output characteristic between the solar cells or partial shade by buildings or the like, the solar power generation system 8 may be unable to operate at the maximum power point.
To cope with this problem, Japanese Patent Laid-Open No. 2000-112545 discloses a solar power generation system in which a DC/DC converter is connected to each solar battery array through a connection box, and all DC output powers are input to an inverter at once and converted into an AC power. According to this arrangement, each DC/DC converter executes maximum power point tracking control for a corresponding solar battery array. Hence, the accuracy of maximum power point tracking control of the solar power generation system increases.
Japanese Patent Laid-Open No. 8-70533 discloses a technique in which an inverter is connected to each solar battery array, each solar battery module, or each solar cell to reduce the output variation between the solar battery modules or solar cells or the power efficiency difference due to partial shade. This arrangement that attaches an inverter to each solar battery module or solar cell can also cope with an increase or decrease in power generation amount by solar batteries at a low cost.
However, these prior arts have the following problems.
In the solar power generation system which inputs the DC output from the solar battery array or solar battery module to the DC/DC converter, as disclosed in Japanese Patent Laid-Open No. 2000-112545 or 6-309047, a solar battery module must be formed by connecting a plurality of solar cells in series.
Generally, formation of solar battery modules is time-consuming because it requires a number of processes including a cutting step of dividing a substrate having an electromotive layer formed thereon into solar cells, an end portion etching step of forming non-power-generation regions that insulate the solar cells from each other, a step of sequentially connecting the solar cells in series by using wiring members such as interconnects, a step of connecting a bypass diode to reduce the influence of partial shade, a step of covering the series-connected solar cells, and a step of fitting a frame into the end portion of the covered structure. In addition, since the members to be used are expensive, the price of solar power generation apparatuses increases.
Especially, when a large solar battery module is to be manufactured, the step of connecting a number of solar cells in series takes a time and labor. This poses a serious problem in manufacturing a large solar battery module.
When the solar cells are to be connected in series using wiring members such as interconnectors, gaps to receive the interconnectors must be formed between the solar cells. The number of gaps increases as the number of series-connected solar cells increases. Hence, the area of the non-power-generation regions in the solar battery module increases. As a consequence, the area power generation efficiency of the solar battery module decreases.
Furthermore, since the solar cells are connected in series, the influence of partial shade on the power generation efficiency is also large. For example, if one of the solar cells connected in series is partially shaded, the amount of current generated by that cell decreases. This cell also rate-determines currents generated by the remaining cells.
To reduce the influence of partial shade, a bypass diode must be connected in parallel with each of the series-connected solar cells. However, even when this method is used, the influence of partial shade on the remaining power generating cells cannot be completely eliminated.
Particularly, as described in Japanese Patent Laid-Open No. 8-70533, when an inverter is connected to each solar cell, labor for the above-described series connection step may be reduced. However, a time and labor are still necessary for the cutting step and etching step for the end portion of each solar cell in forming solar cells.
When the solar cells should be installed on a support, they must be accurately installed at a predetermined interval for the sake of electrical insulation between the solar cells, appearance, and improvement of the area power generation efficiency. However, this work is difficult and increases the cost.
U.S. Pat. No. 4,773,944 discloses a solar battery module which is formed by parallel-connecting all solar cells formed on one substrate. This structure should solve the above-described problems such as the cumbersomeness of the series connection step, the increase in cost, the influence of partial shade, and the difficulty in installation.
In this solar battery module, a bus bar for current collection is connected to the current collection electrodes of the solar cells so that the outputs from the plurality of cells are collected and output.
In this arrangement, however, the current that flows through the current collection bus bar has a value corresponding to the sum of outputs currents of the plurality of cells. Hence, when the number of solar cells increases, and the area of the solar battery module becomes large, the loss in current collection considerably increases.
To solve the problem of current collection loss, the sectional area of the current collection bus bar is increased. However, if this measure is taken, the weight and volume of the current collection bus bar become very large, resulting in difficulty in manufacture and transportation.